1. Field of the Invention
The present invention relates to a portable information apparatus, and more particularly, to a portable information apparatus incorporating a display device using an organic EL (electroluminescence) element, such as a portable telephone, a PDA, a portable personal computer, a portable navigation system, and an electronic book.
Note that, in the present specification, the EL element indicates both an element using light emission (fluorescence) from a singlet exciton and an element using light emission (phosphorescence) from a triplet exciton.
2. Description of the Related Art
In recent years, a portable telephone has become popular by the development of a communication technique. In future, electrical transmission of moving pictures, and transmission of a larger amount of information are expected. On the other hand, a personal computer is also made lightweight, and a product for mobile use is produced. A number of information instruments called personal digital assistants (PDA) starting with electronic notebooks are also produced and are coming into wide use. Further, an EL display device and the like is developed, so that most of the portable information instruments are equipped with flat displays.
Further, in a recent technique, there is a tendency that an active matrix type display device is used as an EL display device employed for those.
In the active matrix type display device, a thin film transistor (hereinafter referred to as a TFT) is arranged for each pixel, and an image is controlled. As compared with a passive matrix type display device, such an active matrix type display device has merits in that high definition can be achieved, picture quality can be improved, moving pictures can be handled, and the like. Therefore, in future, it appears that the EL display device of a portable information apparatus is changed from the passive matrix type to the active matrix type.
Further, in the active matrix type display devices, in recent years, a display device using low temperature polysilicon has been realized as a product. In a low temperature polysilicon technique, in addition to pixel TFTs constituting pixels, a driving circuit can be formed at the peripheral portion of a pixel portion by using TFTs at the same time, which greatly contributes to miniaturization of a device and reduction in consumed electric power. Accordingly, the EL display device becomes an indispensable device for a display portion of a mobile instrument or the like the application field of which is remarkably expanded in recent years.
FIG. 15 is a block diagram of a conventional portable information terminal incorporating an EL display device.
The portable information terminal is required to extract information demanded by user as the need arises. The information is stored in a memory device (a DRAM 1509, a flash memory 1510, etc.) in the portable information terminal, is stored in a memory card 1503 inserted into the portable information terminal, or is obtained through connection to an external instrument via an external interface portion 1505. The information is processed by a CPU 1506 on the basis of instructions of the user inputted from a pen input tablet 1501, and an EL display device 1513 carries out a display.
Specifically, a signal inputted from the pen input tablet 1501 is detected by a detection circuit 1502, and is inputted to a tablet interface 1518. This input signal is processed by the tablet interface 1518, and is inputted to a picture signal processing circuit 1507 and the like. The CPU 1506 processes necessary data, converts it into image data on the basis of an image format stored in a VRAM 1511, and sends it to an EL controller 1512. Here, the EL controller 1512 generates a signal for driving the EL display device 1513 and inputs it to the display device 1513. In this way, the display device 1513 is driven to carry out a display.
FIG. 16 is a block diagram of a conventional portable telephone incorporating an EL display device. The portable telephone includes a transmit-receive circuit 1615, a voice processing circuit 1602 for voice processing a received signal, a speaker 1614, a mike 1608, a keyboard 1601 for inputting data, a keyboard interface 1618 for processing a signal inputted from the keyboard 1601, and the like.
On the basis of the instructions of the user inputted from the keyboard, the CPU 1606 processes information stored in the memory device (a DRAM 1609, a flash memory 1610, etc.), information stored in a memory card 1603 inserted in the portable information terminal, or information obtained through connection to an external instrument via an external interface port 1605, and an EL display device 1613 carries out a display.
Specifically, a signal inputted from the keyboard 1601 is processed by the keyboard interface 1618 and is inputted to a picture signal processing circuit 1607 and the like. A CPU 1606 processes necessary data, converts it into image data on the basis of an image format stored in a VRAM 1611, and sends it to an EL controller 1612. Here, the EL controller 1612 generates a signal for driving the EL display device 1613, and inputs it to the display device. In this way, the display device is driven to carry out a display.
An example of a structure of the transmit-receive circuit 1615 is shown in FIG. 26.
The transmit-receive circuit 1615 includes an antenna 2602, filters 2603, 2607, 2608, 2612, and 2616, a switch 2604, amplifiers 2605, 2606 and 2617, a first frequency changing circuit 2609, a second frequency changing circuit 2613, a frequency changing circuit 2611, oscillation circuits 2610 and 2614, a chopper 2615, a data demodulation circuit 2618, and a data modulation circuit 2619.
Here, as a display device incorporated in the portable information terminal or the portable telephone, a conventional digital system EL display device will be described. FIG. 13 is its schematic view. A pixel portion 1308 is arranged at the center. A source signal line driving circuit 1301 for controlling source signal lines is arranged at the upper side of a pixel portion. The source signal line driving circuit 1301 includes a shift register circuit 1303, a first latch circuit 1304, a second latch circuit 1305, a D/A converter (D/A conversion circuit) 1306, an analog switch 1307, and the like. Gate signal line driving circuits 1302 for controlling gate signal lines are arranged at both sides of the pixel portion. Note that, in FIG. 13, although the gate signal line driving circuits 1302 are arranged at both sides of the pixel portion, they may be arranged at one side. However, the arrangement at both sides is desirable in view of driving efficiency and driving reliability.
The source signal line driving circuit 1301 has a structure as shown in FIG. 14. The driving circuit shown as an example in FIG. 14 is a source signal line driving circuit corresponding to a horizontal resolution of 1024 pixels and a display of 3-bit digital gradation signal, and includes shift register circuits (SR) 1401, first latch circuits (LAT 1) 1402, second latch circuits (LAT 2) 1403, D/A converters (D/A) 1404, and the like. Note that, although not shown in FIG. 14, a buffer circuit, a level shifter circuit, and the like may be arranged as the need arises.
The operation of the display device will be described in brief with reference to FIGS. 13 and 14. First, clock signals (S-CLK, S-CLKb) and a start pulse (S-SP) are inputted to the shift register circuit 1303 (expressed as SR in FIG. 14), and pulses (sampling pulses) are sequentially outputted. Subsequently, the pulses are inputted to the first latch circuit 1304 (expressed as LAT 1 in FIG. 14), and digital signals (Digital Data) inputted to the same first latch circuit 1304 are respectively held. Here, Dl is the most significant bit (MSB) and D3 is the least significant bit (LSB). In the first latch circuit 1304, when holding of the digital signals for one horizontal period is completed, the digital signals held in the first latch circuit 1304 are transferred in the retrace period to the second latch circuit 1305 (expressed as LAT 2 in FIG. 14) all at once in accordance with the input of a latch signal (Latch Pulse).
Thereafter, the shift register circuit 1303 is again operated, and holding of digital signals for a next horizontal period is started. At the same time, the digital signals held in the second latch circuit 1305 are converted into analog signals by the D/A converter 1306 (expressed as D/A in FIG. 14). The analog signals are inputted to the pixels through the source signal lines. This operation is repeated, so that an image is displayed.
Subsequently, driving of the pixel portion 1308 will be described. FIGS. 29A and 29B show a part of the pixel portion 1308 of FIG. 13. FIG. 29A shows a matrix of 3×3 pixels. A portion surrounded by a dotted line frame 1900 is one pixel, and FIG. 29B is an enlarged view thereof. In FIG. 29B, reference numeral 1901 designates a TFT (hereinafter referred to as a switching TFT) functioning as a switching element when a signal is written into the pixel. Any polarity of an N-channel type and a P-channel type may be used for the switching TFT 1901. Reference numeral 1902 designates a TFI (hereinafter referred to as an EL driving TFT) functioning as an element (current control element) for controlling an electric current supplied to an EL element 1903. In the case where the P-channel type is used for the EL driving TFT 1902, it is arranged between an anode 1909 of the EL element 1903 and a current supply line 1907. As another constitution method, the N-channel type is used for the EL driving TFT 1902, and it can also be arranged between a cathode 1910 of the EL element 1903 and a cathode electrode 1908. However, since the grounded source is excellent for the operation of a TFI, and in view of the restriction in the manufacture of the EL element 1903, a system is generally often adopted, in which the P-channel type is used for the EL driving TFT 1902 and as shown in FIG. 29B, the EL driving TFT 1902 is arranged between the anode 1909 of the EL element 1903 and the current supply line 1907. Reference numeral 1904 designates a storage capacitor for holding a signal (voltage) inputted from a source signal line 1906. Although one terminal of the storage capacitor 1904 in FIG. 29B is connected to the current supply line 1907, there is also a case where a dedicated wiring line is used. A gate electrode of the switching TFT 1901 is connected to a gate signal line 1905, one of a source region and a drain region is connected to the source signal line 1906, and the other is connected to a gate electrode of the EL driving TFT.
Next, the operation of a circuit of an active matrix type EL display device will be described with reference to FIGS. 29A and 29B. First, when the gate signal line 1905 is selected, a voltage is applied to the gate electrode of the switching TFT 1901, and the switching TFT 1901 comes to have a conductive state. Then, the signal (voltage) of the source signal line 1906 is inputted to the storage capacitor 1904. Since the voltage of the storage capacitor 1904 becomes a voltage VGS between the gate and source of the EL driving TFT 1902, a current corresponding to the voltage of the storage capacitor 1904 flows through the EL driving TFT 1902 and the EL element 1903. As a result, the EL element 1903 lights up.
The brightness of the EL element 1903, that is, the amount of current flowing through the EL element 1903 is controlled by the voltage VGS of the EL driving TFT 1902. The voltage VGS is the voltage of the storage capacitor 1904, and is the signal (voltage) inputted to the source signal line 1906. That is, by controlling the signal (voltage) inputted to the source signal line 1906, the brightness of the EL element 1903 is controlled. Finally, the gate signal line 1905 is made to have the non-selected state, the gate of the switching TFT 1901 is closed, and the switching TFT 1901 is made to have the non-conduction state. At that time, the electric charge stored in the storage capacitor 1904 is held. Thus, the voltage VGS of the EL driving TFT 1902 is held as it is, and the current corresponding to the voltage VGS continues flowing through the EL driving TFT 1902 to the EL element 1903.
The driving of the EL element etc. is reported in SID99 Digest: P372: “Current Status and future of Light-Emitting Polymer Display Driven by Poly-Si TFT”, ASIA DISPLAY98: P217: “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, Euro Display99 Late News: P27: “3.8 Green OLED with Low Temperature Poly-Si TFT”, and the like.
In the conventional portable information apparatus as described above, in the case where the incorporated display device displays an image, even if the image is a still image, data of the same picture continues to be transmitted to the display device 60 times per second. That is, in FIG. 15, the portions (the picture signal processing circuit 1507 in the CPU 1506, the VRAM 1511, the EL controller 1512, the source signal line driving circuit and the gate signal line driving circuit of the EL display device 1513, the pen input tablet 1501, the detection circuit 1502, and the tablet interface 1518) surrounded by dotted lines continue the operation as long as the image is displayed. Further, in FIG. 16, the portions (the picture signal processing circuit 1607 in the CPU 1606, the VRAM 1611, the EL controller 1612, the source signal line driving circuit and the gate signal line driving circuit of the EL display device 1613, the keyboard 1601, and the keyboard interface 1618) surrounded by dotted lines continue the operation as long as the image is displayed.
Here, in some of the passive matrix type display devices having a small number of pixels, a driver IC of a display device or a controller has a built-in memory circuit, and a VRAM is stopped. However, in the display device using a large number of pixels, such as an active matrix type display device, to incorporate a memory circuit in a driver or a controller is impractical in view of the chip size. Thus, in the conventional portable information apparatus, even in the case where a still picture is displayed, many circuits must continue the operation, which prevents the reduction in consumed electric power.
Further, in a mobile instrument, reduction in consumed electric power is greatly desired. Further, in the mobile instrument, in spite of the fact that it is mostly used in a still picture mode (still picture is continuously displayed), since the driving circuit continues the operation even at the time of displaying the still picture as described above, this is an obstacle to the reduction in consumed electric power.